This invention relates to a method for dry-etching a chromium (Cr) or chromium oxide film, and particularly relates to an etchant gas used in the method for dry etching the film.
Cr thin film is a material which is indispensable for producing the masks used for generating precision patterns in IC chips. Such Cr film is generally subjected to a photo-lithographic process. Recently an electron-beam lithographic process has been used in the place of a photo-lithographic process. Also, it is conventional to selectively etch off using a wet process for developing the final pattern. However, dry etching processes are beginning to be employed instead of the wet process, in order to provide finer patterns and to simplify the etching process of the Cr films.
In the dry etching of Cr film, an etchant gas containing a halide of hydrocarbon, for example, CCl.sub.4, and oxygen is generally used. The etching mechanism involved in the dry etching is considered to be very complicated, but it is believed that the dominant process is a kind of reactive sputtering. The Cr or Cr-oxide film is removed by forming a chromyl halide having a relatively high vapor pressure, such as chromyl chloride (CrO.sub.2 Cl.sub.2) through a chemical reaction with a halogen-containing etchant gas, such as CCl.sub.4, accompanied by oxygen gas. A chemical reaction between the Cr film and the etchant gas may be illustrated by following: EQU 2Cr+CCl.sub.4 +30.sub.2 b =2CrO.sub.2 Cl.sub.2 +CO.sub.2 ( 1)
However, CrO.sub.2 Cl.sub.2 is an unstable compound so the reverse reaction, toward left side in the above equation, can not be neglected. As a result, Cr atoms, released by the decomposition, deposit on the Cr film to be etched. Therefore, the etching rate data for a Cr film is usually small as compared to data relating to other materials such as silicon. As mentioned above the dry etching mechanism is complicated, accordingly the rate of the reverse reaction forming Cr atoms through the decomposition of CrO.sub.2 Cl.sub.2 seems to fluctuate depending upon the reaction conditions. Therefore, extremely precise control of the reaction conditions is required in order to obtain reproducible results relating to the etching rate of the Cr film. The factors which must be controlled in this regard include, among other factors, the partial pressures and flow rates of the respective constituents in the etchant gas, input power for the generating plasma of the etchant gas, and the exhausting velocity of the pumping system in the dry etching system. However, there are further unknown factors which tend to decrease reproducibility.
As described above, the etching rate of a resist film is generally higher than the etching rate of Cr to the etchant gas. Because of this, a resist mask film of a relatively larger thickness is needed for patterning the Cr film. The low etching rate of Cr film results in a long etching time and causes pattern deformation in the masking resist film due to a temperature rise during the etching period. Further, the necessarily large thickness of the resist film inevitably leads to a poor accuracy in the pattern formed in the resist film. These problems are particularly critical in resist films which are sensitive to electron beam exposure, because their chemical and physical stability in dry etching circumference are low compared with photo-sensitive resist film.